Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements –Part 1: Methodology and evaluation with simulated measurements

[摘要] We present a new remote sensing technique to infer the averageasymmetry parameter of ice crystals near cloud top frommulti-directional polarization measurements. The method is based onprevious findings that (a) complex aggregates of hexagonal crystalsgenerally have scattering phase matrices resembling those of theircomponents; and (b) scattering phase matrices systematically varywith aspect ratios of crystals and their degree of microscalesurface roughness. Ice cloud asymmetry parameters are inferred frommulti-directional polarized reflectance measurements by searchingfor the closest fit in a look-up table of simulated polarizedreflectances computed for cloud layers that contain individual, randomly orientedhexagonal columns and plates with varying aspect ratios androughness values. The asymmetry parameter of the hexagonal particlethat leads to the best fit with the measurements is considered theretrieved value. For clouds with optical thickness less than 5, thecloud optical thickness must be retrieved simultaneously with theasymmetry parameter, while for optically thicker clouds theasymmetry parameter retrieval is independent of cloud opticalthickness.Evaluation of the technique using simulated measurementsbased on the optical properties of a number of complex particles andtheir mixtures shows that the ice crystal asymmetry parameters aregenerally retrieved to within 5%, or about 0.04 in absoluteterms. The retrieval scheme is largely independent of calibrationerrors, range and sampling density of scattering angles and randomnoise in the measurements. The approach can beapplied tomeasurements of past, current and future airborne and satelliteinstruments that measure multi-directional polarized reflectances ofice-topped clouds.

[学科分类] 几何与拓扑